Primary vestibular afferent activity of the posterior semicircular canal of the toadfish will be recorded with a hook electrode or a glass microelectrode. The effects of the efferent vestibular system (electrically activated) will be assessed upon the ongoing spontaneous activity and naturally evoked activity of canal afferents. We will be cognisant of possible differential effects upon tonic and phasic afferents. Receptor-receptor interaction between various vestibular sensory areas and the posterior canal will be assessed by stimulation of each of these areas during recording of afferent activity from the posterior canal. The efferent adrenergic innervation of the labyrinth, described in our laboratory, will be further investigated. Efferent adrenergic receptors will be activated by bath application of noradrenalin and/or electric stimulation of the sympathetic system. Finally, intralabyrinthine dendritic morphology of posterior semicircular canal afferents will be elucidated by an approach employing intracellular labeling with horseradish peroxidase and subsequent reconstruction of labeled fibers using a drawing tube technique. These studies will correlate the dendritic morphology of the primary afferents with the physiological activity of the same afferents previously recorded. The mode of impulse initiation in these same phasic and tonic fibers will be examined.